Intrusion of the maxillary posterior teeth with a single buccal mini-implant positioned bilaterally in young adults with a tendency towards hyperdivergence: A clinical study

Abstract

Background:

Skeletal anchorage systems have been used for intrusion of the posterior teeth with satisfactory results. To achieve this, mini-implants are placed at anatomically challenging sites such as the palate or require several mini-implants to produce the desired effect.

Objective:

To determine the magnitude of intrusion of the maxillary posterior teeth achieved on a continuous arch wire using a single buccal mini-implant placed bilaterally in young patients with a tendency towards hyperdivergence and to evaluate its influence on the skeletal, dental and soft-tissue structures.

Methods:

A total of 17 patients with proclination of the anterior teeth, tendency towards hyperdivergence and clockwise rotation of the mandible were selected. First premolars were extracted as part of treatment protocol. A 0.022-MBT bracket prescription was used. Mini-implants were placed bilaterally on the buccal aspect at the mucogingival junction or slightly gingival to it between the maxillary second premolar and first permanent molar. A total of 200 g of intrusive force was placed from a continuous 0.019 × 0.025 inch stainless-steel arch wire to the mini-implant by means of an elastomeric thread on both sides. Lateral cephalograms and study models were taken before the start of intrusion and six months later. Parametric and non-parametric tests were done to assess treatment results.

Results:

Significant intrusion was observed in the maxillary molar and premolar region with tendency towards intrusion in the anterior region. There was significant decrease in lower anterior facial height (LAFH) with anti-clockwise mandibular rotation, decrease in facial proportion index and total facial height. No changes were observed in the transverse plane.

Conclusion:

Intrusion of the permanent maxillary molar can be achieved on a continuous arch wire with a single buccal mini-implant placed bilaterally with improvement in facial aesthetics, especially in the vertical plane. This method may be beneficial in patients with borderline vertical discrepancy treated with conventional friction mechanics during space closure after first premolar extractions.

Keywords

intrusion posterior teeth mini-implant

Introduction

Management of vertical skeletal discrepancies in an adult patient is a very challenging scenario. Treatment plans often involve orthodontic treatment combined with or without orthognathic surgery to correct the dental and skeletal compensations and bring about a considerable improvement in facial appearance. Intrusion of the posterior dentition either in the maxillary or mandibular arch will usually result in a counter-clockwise mandibular rotation, reduction in lower anterior facial height (LAFH) with improvement in facial profile (Alsafadi et al., 2016) in mild to moderate cases. It is difficult to achieve intrusion of the posterior teeth with conventional orthodontic treatment mechanics alone. With the advent of mini-implants, intrusion of the posterior dentition can be obtained with favourable results. Based on the envelope of discrepancy, the amount of tooth movement that can be obtained with orthognathic surgery is significantly greater than that obtained with mini-implants (Proffit and Fields, 2012). Therefore, treatment with mini-implants may not produce a dramatic change compared to orthognathic surgery, but a considerable improvement in facial appearance may be obtained.

Intrusion of the posterior dentition has been attempted with mini-implants, bone plates and other devices with variable results (Alsafadi et al., 2016; Argumedo et al., 2014; Carano et al., 2005; Erverdi et al., 2004; Hart et al., 2015; Kuroda et al., 2007; Park et al., 2003; Sherwood et al., 2002; Xun et al., 2007). In the maxillary arch, mini-implants are routinely placed on the palatal aspect for intrusion of the arch with satisfactory results (Alsafadi et al., 2016; Hart et al., 2015; Xun et al., 2007). However, placement of a mini-implant on the palatal side requires greater expertise compared to the buccal side. Reducing the number of mini-implants is also beneficial in reducing patient anxiety and surgical morbidity. In this study an attempt has been made to intrude the posterior teeth with a single buccal mini-implant positioned bilaterally.

Prevalence of vertical discrepancy and tendency towards hyperdivergent facial pattern is in the range of 10%–34.94% among different populations (Cardoso et al., 2011). These patients may not warrant orthognathic surgery but may benefit from intrusion of the posterior teeth with improvement of facial aesthetics. Hence, the present study was proposed to evaluate the possibility of intrusion of the posterior teeth on a continuous arch wire with a single mini-implant placed bilaterally on the buccal aspect in the maxillary posterior region in patients with mild to moderate vertical discrepancy.

The primary aim of the present study was to determine the amount of maxillary molar intrusion that can be obtained with a single buccal mini-implant during conventional orthodontic treatment with friction mechanics in young adults with a tendency towards hyperdivergence. The secondary aim was to assess whether the change in molar position had an effect on the skeletal, dental and soft-tissue structures.

The null hypothesis was that there is no intrusion of the maxillary molar when an intrusive force is applied from a single buccal mini-implant positioned bilaterally during conventional orthodontic treatment in young adults with a tendency towards hyperdivergence.

Methods

This was a single-centre prospective clinical trial. The study was approved by the scientific review board of our university and further reviewed and approved by the Institutional Human Ethics Committee of our university (reference IHEC/SDMDS13ORT1). The study was carried out in the university orthodontic department.

The sample consisted of 17 participants. The sample size was calculated for the primary objective, namely molar intrusion using G power software based on existing literature (Erverdi et al., 2004). Calculation of the sample size a priori with a correlation between groups set as 0.5 showed that for a power of 95%, 14 patients need to be recruited with an effect size of 1.088. The smallest change in the intrusion of the molar was set at 1 mm and was considered the minimal clinical important difference. A unit change in all the other parameters was considered to be clinically significant. A total of 19 patients (9 men, 10 women) were included in the study to avoid loss due to follow-up (attrition bias). Two female patients dropped out over the course of the study due to personal reasons.

Inclusion criteria consisted of patients aged 16–35 years belonging to both sexes with a tendency towards hyperdivergence, increased facial proportion index (FPI) and proclination of the anterior teeth requiring bilateral first premolar extraction in both arches. All patients were in the initial levelling and aligning phase of orthodontic treatment without any history of temporomandibular joint (TMJ) problems or any previous history of orthodontic treatment.

Exclusion criteria were as follows: individuals aged <15 years and >35 years; those who had already completed orthodontic treatment with relapse; periodontally compromised cases; patients with systemic illness; and pregnant women. Patients with GoGn to SN plane ⩾38° were considered to have a severe skeletal discrepancy and were excluded from the study. Patients not belonging to the South Indian population were also excluded from the study.

The baseline data of the patients included in the study are given in Table 1. All patients had a skeletal class I malocclusion with Angle’s class I molar relation tendency towards hyperdivergent growth pattern as shown by the basal plane angle (GoGn and palatal plane) and mandibular plane angle (GoGn to SN plane). All patients had an increased LAFH and proclination of the upper anterior teeth. This was correlated with clinical appearance of the patients. Only young adults aged 16–28 years were selected. Patients aged <15 years were excluded due to the possibility of an immature bone at the site of mini-implant placement which would pose a risk of mini-implant failure (Miyawaki, 2003; Moon et al., 2008). Older patients were also excluded as they have a greater susceptibility to periodontal breakdown.

Table 1.

Baseline data of the patients included in the study.

N	Age	Basal plane angle (Go-Gn to palatal plane in degrees)	Mandibular plane (GoGn to SN in degrees)	FPI (%)	Proclination (U1 TO PP in degrees)	LAFH (mm)
17	Males: 21 ± 1.26 years, Females: 18 ± 0.91 years	30.82 ± 4.41	34.65 ± 5.54	13.72 ± 3.91	65.24 ± 7.26	75.82 ± 6.86

Values are given as mean ± SD.

FPI, facial proportion index; LAFH, lower anterior facial height.

After a detailed explanation of the procedure, written informed consent was obtained from all the patients who matched the inclusion criteria and agreed to participate. All doubts of the patients regarding the study were clarified.

Orthodontic treatment was initiated with fixed orthodontic treatment using a prescription of 0.022 MBT (3M Unitek) in all patients (Figure 1a–1g). All patients had proclination of the upper and lower anterior teeth and required extraction of the first premolars bilaterally. A transpalatal arch was inserted in all patients to prevent buccal flaring of teeth. To avoid impingement of the soft tissue during intrusion, the transpalatal arch was placed slightly away from the palate. The second molar was fully erupted in all patients and was incorporated onto the arch wire in all patients. Levelling and aligning were carried out sequentially using up to 0.019 × 0.025 inch stainless-steel wire in all patients. A mild accentuated curve was incorporated in the arch wire before the placement of intrusive force on the posterior teeth. Pre-intrusion records, namely lateral cephalogram and study models (T1), were taken.

Mini-implants (1.2 × 8 mm) were placed bilaterally with a stent (Felicita, 2013), at the mucogingival junction or slightly gingival to it between the second premolar and first permanent molar at an angulation of 30° to the long axis of the tooth. An elastomeric thread was used to apply the intrusive force from the mini-implants to the base arch wire bilaterally such that they delivered 200 g of force per side (Figure 2a and 2b). A Dontrix gauge was used to measure the amount of intrusive force. The elastomeric thread was tied into a knot. The end of the knot was tightened with a ligature wire onto the base arch wire until the desired force was achieved. The maxillary anterior teeth were not retracted during the first six months of intrusion. This was done to prevent the influence of retractive force on the base arch wire when intrusion was being attempted (performance bias). At the end of six months, the base arch wire was ligated with mini-implant by means of ligature wire to avoid relapse and retraction of the anterior teeth was initiated by means of a closed coil spring from the maxillary first permanent molar to an attachment on the arch wire placed between the lateral incisor and canine on both sides. Patients were reviewed periodically once every three weeks for six months. All the patients were treated by a single operator and all the mini-implants were placed by the same person, a senior postgraduate who was well versed with the technique (operator bias).

At the end of the six-month follow-up period (T2), post-intrusion lateral cephalograms and study models were taken were taken.

The transverse changes in the maxilla were measured on dental casts using a divider and scale. Inter-canine, inter-premolar and inter-molar widths were measured bilaterally between the cusp tip of the maxillary canine, the mesial pit of the maxillary second premolar and the central pit of the maxillary first permanent molar.

Measurements on the lateral cephalograms and study models taken at T1 and T2 were made by a third operator. Blinding of the measurements data was achieved (detection bias). The skeletal, dental and soft-tissue parameters were measured. The values obtained were subjected to statistical evaluation.

All statistical analyses were performed using SPSS version 22 (IBM Corp., Armonk, NY, USA). The Shapiro–Wilk test did not show a normal distribution for the cephalometric values measured; hence, the Wilcoxon signed rank test was performed. Bonferroni correction was carried out and the level of significance was set at 0.05/31=0.0016 as 31 parameters were evaluated to assess overall treatment outcome. Paired t-test was performed for the transverse measurements made on the study models.

Results

The results obtained from the statistical evaluation are given in Tables 1–4. The mean age of the male patients was 21 ± 1.26 years and 18 ± 0.91 years in female patients. Hyperdivergence was confirmed with the measurement of basal plane angle (GoGn to PP) and mandibular plane (GoGn and SN) (Table 1). Since retraction of the maxillary anterior teeth was a prerequisite in all cases, only those with proclination of the upper anterior teeth were selected (Table 1).

The null hypothesis was rejected. Evaluation of treatment results at the end of intrusion showed no significant change in the sagittal skeletal relation of the maxilla and mandible as shown by the values of SNA, SNB and ANB. However, in the vertical plane an anti-clockwise mandibular rotation with reduction in basal plane angle (GoGn to palatal plane) and mandibular plane angle (GoGn to SN plane) was noted and this was statistically significant. A statistically significant reduction in LAFH, total anterior facial height and FPI was also noted (Table 2) and this resulted in an improvement in facial aesthetics in the vertical plane (Figure 3a–3g).

Table 2.

Comparison of treatment changes (T1-T2): skeletal.

Parameter	Pre-intrusion (T1)	Post-intrusion at the end of six months (T2)	Difference T2–T1	Z	Asymp Sig. (two-tailed)
SNA(°)	85.29 ± 3.84	85.24 ± 4.32	–0.06 ± 0.97	–0.250*	0.803
SNB(°)	77.94 ± 3.29	78.29 ± 3.46	0.35 ± 1.06	–1.396^†	0.163
ANB(°)	7.47 ± 2.003	7.23 ± 2.39	–0.24 ± 1.20	–0.855*	0.392
GoGn to PP(°)	30.82 ± 4.41	29.41 ± 4.64	–1.41 ± 1.46	–2.858*	0.004^‡
GoGn to SN(°)	34.65 ± 5.54	33.47 ± 5.47	–1.18 ± 0.81	–3.272*	0.001^‡
Angle of inclination	90.18 ± 4.23	90.53 ± 3.502	0.35 ± 2.09	–0.108^†	0.914
UFH (mm)	51.53 ± 3.29	51.35 ± 3.32	–0.18 ± 0.64	–1.134*	0.26
LAFH (mm)	75.82 ± 6.86	74.71 ± 6.84	–1.12 ± 0.99	–3.002*	0.003^‡
FPI	13.72 ± 3.91	12.26 ± 4.01	0.54 ± 0.65	–2.533^†	0.011^‡
PFH (mm)	82.41 ± 7.96	82.53 ± 7.27	0.12 ± 1.36	–0.416^†	0.677
TAFH (mm)	126.88 ± 8.56	125.47 ± 8.54	–1.41 ± 1.12	–3.111*	0.002^‡
Ar-Go-Me(°)	129.88 ± 6.79	129.82 ± 6.73	–0.56 ± 0.97	–0.187*	0.851
N-S-Ar(°)	113.65 ± 26.41	120.24 ± 4.19	6.59 ± 2.43	–0.934^†	0.35

Values are given as mean ± SD.

Based on positive ranks.

†

Based on negative ranks.

‡

Statistically significant.

There was significant intrusion of the maxillary premolar and first permanent molar and this contributed to the overall vertical change. The maxillary incisors showed a tendency towards intrusion and this was statistically significant. The amount of maxillary molar intrusion achieved was 1.47 ± 0.8 mm and this was greater than the minimal clinically important difference. The maxillary incisors showed a labial inclination of 0.47° ± 3.17° and the maxillary first permanent molar showed a distal inclination of 1.53° ± 1.01°. This was statistically significant and was probably due to the accentuated curve placed in the arch wire. Overbite changed from 2.59 ± 1.58 mm to 2.65 ± 1.66 mm with a mean increase of 3 ± 1.39 mm, although it was not statistically significant. An improvement in inter-incisal angle was also observed. Although intrusion was performed on a continuous arch wire, canting of the occlusal plane was not observed (Table 3).

Table 3.

Comparison of treatment changes (T1–T2): dental and soft tissue.

Parameter	Pre-intrusion(T1)	Post-intrusion at the end of six months (T2)	Difference T2–T1	Z	Asymp Sig. (two-tailed)
Dental
U1 TO PP (°)	64.18 ± 7.76	65.24 ± 7.26	0.47 ± 3.17	–2.62*	0.009^†
U5 TO PP (°)	86.29 ± 16.07	89.76 ± 4.10	3.47 ± 14.61	–0.36^‡	0.716
U6 TO PP (°)	90.00 ± 3.32	88.47 ± 3.59	–1.53 ± 1.01	–3.35*	0.001^†
U1_Cres TO PP (mm)	18.88 ± 3.22	17.82 ± 3.13	–0.24 ± 1.52	–2.97*	0.003^†
U5_CresTo PP (mm)	27.59 ± 3.22	27.06 ± 3.17	–1.65 ± 0.93	–3.46*	0.001^†
U6_CresTo PP (mm)	28.71 ± 3.29	26.12 ± 3.18	–1.47 ± 0.8	–3.49*	0.000^†
U6 TO pterygoid vertical (mm)	24.18 ± 2.88	24.47 ± 2.94	0.29 ± 1.05	–1.22^‡	0.222
Upper OP TO PP (°)	11.24 ± 4.59	11.47 ± 3.91	0.24 ± 1.48	–0.75^‡	0.452
Lower OP TO MP (°)	22.65 ± 5.58	22.94 ± 5.56	0.29 ± 1.312	–0.89^‡	0.371
Interincisal angle (°)	110.82 ± 14.24	109.94 ± 14.05	0.65 ± 3.62	–2.01*	0.044^†
Overjet (mm)	4.76 ± 1.71	4.59 ± 1.84	–0.18 ± 0.81	–0.97*	0.334
Overbite (mm)	2.59 ± 1.58	2.65 ± 1.66	3 ± 1.39	–0.28*	0.776
Soft tissue
Upper lip to E plane (mm)	2.41 ± 2.79	2.06 ± 2.33	–0.35 ± 1.22	–1.15*	0.25
Lower lip to E plane (mm)	6.94 ± 4.41	6.12 ± 4.08	–0.82 ± 1.98	–1.79*	0.072
Nasolabial angle (°)	95.82 ± 11.07	95.35 ± 10.93	0.47 ± 1.84	–0.99*	0.322

Values are given as mean ± SD.

Based on positive ranks.

†

Statistically significant.

‡

Based on negative ranks.

There was no significant change in cephalometric measurements of the soft tissue as retraction of the anterior teeth was not performed (Table 3). There was no change in the transverse dimension of the maxillary arch (Table 4).

Table 4.

Comparison of transverse changes: cast analysis.

Parameter	Pre-intrusion (T1)	Post-intrusion at the end of six months (T2)	t value	P value
Intercanine width (mm)	34.1 ± 0.34	34.2 ± 0.43	0.390	0.702
Interpremolar width (mm)	36.1 ± 0.41	35.8 ± 0.46	–1.765	0.097
Intermolar width (mm)	46.9 ± 0.66	45.9 ± 0.84	–1.580	0.134

Discussion

This clinical study was conducted with strict adherence to ethical guidelines. Patients with tendency towards hyperdivergence with a clockwise rotation of the mandible were selected. Only young adult patients were chosen to minimise the effect of growth and failure of the mini-implants due to poor bone quality (Buschang et al., 2011; Rice et al., 2019). Intrusion of the posterior teeth was required in all patients. Various methods have been reported in the literature to achieve intrusion. Molar intrusion in the maxillary arch may be performed with bone plates (Erverdi et al., 2004; Sherwood et al., 2002), infrazygomatic implants (Almeida et al., 2019; Flieger et al., 2012), palatal mini-implants (Wilmes et al., 2020) or a combination of buccal and palatal mini-implants (Paccini et al., 2016; Yao et al., 2005) and other conventional methods (He et al., 2013; Tabancis et al., 2020). We chose a single buccal mini-implant per side in the current study to keep the number of mini-implants to a minimum and still consider the possibility of intrusion of the maxillary molars. Retraction of the upper anterior teeth was not performed simultaneously along with intrusion of the posterior of the teeth. This was done to eliminate the effects of the retractive forces on the posterior teeth. All the mini-implants were placed at the mucogingival junction or slightly gingival to it. Placing the mini-implant perpendicular to the buccal surface would place the mini-implant at the mid-section of the root. This can result in contact of the root with the mini-implant. Angulating the mini-implant will place it more towards the apex of the tooth, reducing the risk of mini-implant failure due to root contact during intrusion. Placing the mini-implant more towards the root apex increases the possibility of sinus perforation. In addition, angulating the mini-implant increases the area of contact with the cortical bone improving the stability of the mini-implant (Laursen et al., 2013).

Care was taken to minimise unwanted and undesirable tooth movement. Elastomeric thread was used to apply a mild intrusive force from the mini-implant on the posterior teeth through the base arch. The intrusive force was increased to 200 g at each sitting since elastomeric thread has a force degradation of 65% at the end of three weeks (Persson et al., 1983). Root resorption was not assessed.

With this method a vertical bowing effect can be expected in the posterior region since the force from the mini-implant is applied at a single point on the base arch wire. However, this was not seen. This might be due to two factors: first, the small inter-bracket distance between the maxillary second premolar and the first permanent molar that decreased the load deflection rate and prevented upward bending of the arch wire; and second, the use of a stiff 0.019 × 0.025 inch stainless-steel base arch wire with a mild accentuated curve. The deflection in the arch wire due to the accentuated curve placed in it may have negated the tendency towards lateral bite opening in the posterior region. Placement of an accentuated curve may have only a minimal effect on the intrusion of the maxillary teeth. The effects of the accentuated curve placed in the arch wire have been evaluated in the literature (Bernstein et al., 2007). During conventional orthodontic mechanics an accentuated curve placed in the arch wire may cause extrusion of the premolars but has minimal intrusive effect on the incisors (Bernstein et al., 2007). In the present study, the extrusion of the premolars did not occur probably due to the force applied on the mini-implant. Although the accentuated curve placed in the arch wire may facilitate bite opening in the anterior region, they do not cause true intrusion of the maxillary incisors (Bernstein et al., 2007). The combined effects of the mild accentuated curve in the upper arch along with a posterior intrusive force in the present study may be the reason for the tendency towards incisor intrusion although no intrusive force was placed in the anterior region.

Since the force from the mini-implant is placed buccal to the centre of resistance of the maxillary posterior teeth, buccal flaring of the posterior teeth can be expected. This was avoided with the judicious use of the transpalatal arch. The transpalatal arch was placed slightly away from the palatal surface to avoid soft-tissue impingement during the intrusive process. Some may say that such a placement may itself produce intrusion of the molar teeth. However, there is no scientific evidence to indicate that a simple transpalatal arch without any modifications can produce intrusion of the maxillary molars. Intrusion of the posterior teeth with a transpalatal arch is possible only with additional modifications such as addition of acrylic button, loops and other methods (Khan et al., 2020; Kumar et al., 2014; Maurya et al., 2020; Yañez-Vico et al., 2017).

Care should be taken to place the mini-implant at the mucogingival junction or higher, since root contact during intrusion may result in loosening and failure of the mini-implant. The success rate in this study was 95%.

Although statistically significant, the mean distal tipping that occurred was small, approximately 1.53° ± 1.01°. This is because the intrusion was performed on a stiff 0.019 × 0.025 inch stainless-steel wire with a transpalatal arch given in all patients at the start of treatment and the second molars were also included in the arch. This helped minimise the distal tipping of the molar.

This study was constituted to determine if a single mini-implant placed on the buccal aspect on each side between the maxillary second premolar and first permanent molar has an intrusive effect on the posterior teeth and brought about change in facial appearance that may be considered beneficial in patients with mild vertical discrepancy. It is clear that the maxillary posterior teeth have intruded bringing about an anti-clockwise mandibular rotation. There was a reduction in LAFH and FPI with improvement in facial appearance in the vertical plane. Hence, this method appears to be effective in patients with borderline hyperdivergence.

Now, similar intrusion can be achieved with other methods such as mini-plates, mini-implant at other sites, multi-looped arch wire, magnets, posterior bite blocks and surgical procedures. Therefore, it becomes imperative to compare the treatment results from the current study with those achieved with other treatment mechanics.

Mini-plates produce greater intrusion of the posterior teeth compared to mini-implants. Molar intrusion achieved with mini-plates is in the range of 1.99–2.6 mm with an anti-clockwise mandibular rotation by 1.7°–2.62° and reduction in LAFH by 3–3.6 mm (Erverdi et al., 2004; Kuroda et al., 2007; Sherwood et al., 2002; Turkkahraman and Sarioglu, 2016). In the present study, the magnitude of maxillary molar and premolar intrusion was 1.65 ± 0.93 mm and 1.47 ± 0.8 mm, respectively. This was definitely less than those achieved with mini-plates. Although the intrusion achieved was greater with mini-plates compared with mini-implants, an additional surgical procedure is required for the placement/removal of the mini-plates and may be warranted in severe vertical discrepancies. The greater amount of intrusion achieved with mini-plates is probably due to the fact that it is placed at a higher level in the basal bone with an increased range of applied force. Mini-implants have been used by several researchers in the past to bring about intrusion of the molars. The site and the number of mini-implants differ among various studies (Argumedo et al., 2014; Hart et al., 2015; Park et al., 2003, 2006; Xun et al., 2007). Mini-implants placed at sites other than the one chosen in the study produced molar intrusion that was in the range of 1.8–2.3 mm (Baumgaertel et al., 2016; Cousley, 2010, 2014; Hart et al., 2015; Park et al., 2003; Xun et al., 2007). The LAFH reduced by 1.5–1.6 mm (Hart et al., 2015; Xun et al., 2007) and there was an anti-clockwise mandibular rotation by 2.3°. This was comparable with the results of our present study. However, placement of a palatal mini-implant requires greater expertise compared to buccal mini-implants (Baumgaertel et al., 2016; Wilmes et al., 2020). In addition, several mini-implants are required to produce intrusion when both buccal and palatal mini-implants are used (Paccini et al., 2016). A single mini-implant such as the one used in the present study is preferable to several mini-implants to produce the same results.

Non-compliant treatment (Carano et al., 2005) with rapid molar intrusion appliance produced molar intrusion of 2.42 mm and decreased LAFH of 3.15 mm with anti-clockwise mandibular rotation of 2.34°. This was greater than the results of the present study probably because the entire maxillary arch is not bonded during the placement of rapid molar intrusion appliance.

In surgically treated patients there is 7-mm decrease in overbite, 4-mm decrease in the overall facial height and 3.6-mm molar intrusion was achieved (Erverdi et al., 2004; Sherwood et al., 2002). This is definitely greater than those achieved with mini-implants. However, the morbidity of the surgical procedure may warrant its use in patients with severe vertical discrepancy. Extrusion of the incisors was noted in the surgical group (Erverdi et al., 2004; Kuroda et al., 2007).

There is paucity of literature in recent years with the use of magnets and posterior bite blocks for molar intrusion (Arat et al., 2006; Kuster and Ingervall, 1992; Woods and Nanda, 1988). A recent article showed molar intrusion with the use of bonded resin blocks (Vela-Hernández et al., 2017). The amount of molar intrusion was approximately 1 mm (Arat et al., 2006).

Although a multi-looped arch can be used for intrusion or extrusion of the molars along with distal tipping of the molar, sufficient literature is not available to predict the amount of molar intrusion achieved with these mechanics, especially in the maxillary arch. Multi-looped arch wire produced intrusion of the mandibular molars by 0.4 mm (He et al., 2013). Mousetrap appliance brought about 2 mm of molar intrusion (Wilmes et al., 2013) and infrazygomatic implant produced molar intrusion of about 4 mm (Flieger et al., 2012).

Thus, a single mini-implant placed bilaterally in the posterior region may be considered to bring about intrusion of the posterior teeth in the maxillary arch with improvement in facial aesthetics in patients with tendency towards skeletal open bite and borderline hyperdivergent patients.

A limitation of the study would be the absence of a control group. There was no control group as enrolling an adequate number of patients who would meet the selection criteria for both the groups was found to be quite challenging. A controlled clinical trial may be contemplated in the future. Retraction of the upper and lower anterior teeth concomitant with intrusion of the posterior teeth will give a better understanding of the overall treatment results at the end of orthodontic treatment.

Clinically, the mechanics used in this study may benefit patients with a borderline vertical discrepancy where space closure is performed with friction mechanics on a continuous arch wire for retraction of the maxillary anterior teeth into the first premolar extraction space.

Conclusion

The null hypothesis was rejected. Significant intrusion of the permanent first molar and second premolar was noted in the maxillary arch. The amount of molar intrusion achieved was 1.47 ± 0.8 mm. There was a favourable change in skeletal and dental parameters, especially in the vertical plane. The skeletal change that was observed was an anti-clockwise mandibular rotation with decrease in LAFH, total facial height and FPI. The dental change that had clinical relevance apart from intrusion of the maxillary molars was intrusion of the maxillary premolars. Thus, this method may be beneficial in patients with borderline hyperdivergence/vertical discrepancy to bring about an improvement in facial appearance when the friction method is employed for correction of axial inclination of the incisors.

Footnotes

Acknowledgements

We would like to thank all the patients who participated in the study.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

A Sumathi Felicita

Availability of data and material

Availability of data and material can be obtained from the author on request

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